Seismic Swarm PS20210729.1 Near Perryville, Alaska

The seismic swarm designated PS20210729.1 occurred approximately 114 km southeast of Perryville, Alaska, within the tectonically active Aleutian subduction zone. This region marks the convergent boundary where the Pacific Plate subducts beneath the North American Plate at rates exceeding 6 cm per year, generating frequent megathrust earthquakes and associated aftershock sequences. The swarm initiated at 06:15 UTC on 29 July 2021 and concluded at 03:52 UTC on 30 July 2021, spanning 21 hours and 36 minutes during which 11 earthquakes were recorded.

The sequence began with a magnitude 8.2 event at 35 km depth, followed by events ranging from magnitude 5.0 to 5.9 at depths between 0 and 43 km. Notable activity included a magnitude 5.9 shock at shallow depth shortly after the mainshock, along with several magnitude 5.3–5.7 events clustered in the initial hours. Later shocks on 29 July and the final magnitude 5.5 event early on 30 July reflected ongoing stress adjustment along the plate interface.

This swarm is directly linked to the 2021 Chignik earthquake (M 8.2), whose epicenter lay 62 km from the swarm centroid. The mainshock ruptured a segment of the megathrust previously identified as a seismic gap, releasing accumulated strain built up since earlier historic ruptures. The Aleutian arc has produced multiple great earthquakes in the past century, including events in 1938, 1957, and 1964, underscoring the zone’s capacity for M 8+ earthquakes.

Historical records since 2000 indicate only two swarms in this sector, with the preceding episode occurring in 2020. These swarms typically represent aftershock sequences or triggered seismicity rather than independent foreshock activity. Depths recorded during the 2021 swarm, predominantly between 18 and 43 km, align with the expected range for interface and intraslab events in this subduction setting.

The geological framework features the eastward continuation of the Aleutian trench, with Perryville situated on the Alaska Peninsula near the transition from oceanic to continental subduction. Volcanic arcs and accretionary complexes dominate the surface geology, while the subsurface is characterized by a well-defined Wadati-Benioff zone extending to depths greater than 200 km. Updated seismic monitoring by regional networks confirms persistent low-level activity consistent with ongoing plate convergence.

References
USGS Earthquake Catalog (2021 Chignik event details)
Alaska Earthquake Center historical swarm records
SeismoSight internal swarm classification PS20210729.1